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Chemical and isotopic measurement of water and recent sediments at the Plitvice Lakes

Tufa is formed by calcium carbonate precipitation from surface water by a combination of physico-chemical and biological processes. In the Plitvice Lakes the process of tufa formation is very intensive, forming a series of tufa waterfalls and barriers. At the same time the precipitation of lake sediment at the bottom of the lakes is also occurring. Calcium carbonate precipitation in form of tufa or lake sediment is a very sensitive process and any pollution of water and/or changes in the lake environment can disturb the equilibrium conditions of calcite precipitation. To study the physico-chemical conditions of tufa precipitation in the lake waters, the environmental conditions in the Plitvice Lakes and possible anthropogenic influence on the system the following investigations were performed:  Physico-chemical conditions for calcite precipitation. Water samples were collected in different seasons at 18 sampling points including springs, lakes, tributaries and Korana River which outflows from the Plitvice Lakes (total distance ~ 12 km). Some of the sampling points are characterized by the process of eutrophication in the form of intense plant growth in the lakes. Temperature, pH values, conductivity and concentration of dissolved oxygen were measured in situ and in the laboratory alkalinity, CO2, dissolved organic carbon (DOC), kations Ca2+, Mg2+, Na+, K+, Sr2+, anions Cl-, F- and SO42-, and nutrients (ammonium, nitrates). Physico-chemical conditions for calcite precipitation showed that calcite precipitated in lake waters oversaturated with respect to CaCO3 (Isat values 4-10). The present status of the water quality was compared with the parameters measured at the same locations in the period 1982-1985. Most of the parameters did not change significantly. Regarding to all of the water analyses we can conclude that the Plitvice Lakes waters are very clean and oligotrophic with low primary productivity. Local anthropogenic pollution has not been observed ; however, some influence of global climate change has been noticed.  Recent sediments, five short cores (top 40 - 45 cm of sediment) from four lakes of the Plitvice Lakes system were measured for 210Pb, 137Cs, a14C, 13C, 18O to study the influence of environmental changes on the sediment system in small and large lakes. Lake sediments consisted mainly of autochthonous carbonates with higher sedimentation rate in small lakes. Sediments from the two large lakes, Prošće and Kozjak, showed constant stable isotope profiles for the carbonate fraction and full agreement between the 137Cs and 210Pb chronologies. Sediments from the two small lakes, Gradinsko and Kaluđerovac, showed synchronous increases in 14C and 13C and disturbed 137Cs records. All lakes showed an increase in a14C in the carbonate sediments above the first occurrence of 137Cs which was interpreted as a damped and decades-delayed consequence of the bomb-induced increase in a14C in atmospheric CO2. For the small lakes, increased 13C in the last two decades and part of the a14C increase is probably due to an increase in primary productivity that enhanced biologically-induced calcite precipitation with concomitant changes in the carbon isotopic composition of carbonate sediments.  Recent sediments in Lake Kozjak were used to study the impact of the material delivered to the lake sediment via Rječica Brook. Distribution of organic matter, C/N values, 13Corg and 13Ccarb in sediment profiles showed strong influence of terrestrial organic and carbonate fraction run-off via Rječica Brook which decreased rapidly in the direction of the open lake. The sedimentation rate as well as the ratio of allogenic and authigenic fraction in sediments was estimated. The steady decrease of the 13Corg with decreasing depth in all sediment profiles might be a consequence of the increase in algal productivity, but might also be a consequence of the change of 13C of atmospheric CO2 on the global scale due to enhanced fossil fuel combustion during last century.

Plitvice Lakes; recent sediment; water; chemical and isotopic analyses

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